This invention was made with support from the government of the United States of America. The government of the United States of America may have certain rights in this invention.
1. Field of Invention
The present invention relates generally to a corrosion resistant coated fuel cell bipolar plate and a method for making the same and, more specifically, to a coated, corrosion resistant coated fuel cell bipolar plate with filled-in fine scale porosities and a method of making the same.
2. Description of the Related Art
Fuel cells such as the Proton Exchange Membrane (xe2x80x9cPEMxe2x80x9d) fuel cell include a membrane electrode assembly (xe2x80x9cMEAxe2x80x9d). The MEA comprises a solid polymer electrolyte or ion exchange membrane positioned between an anode and a cathode which typically comprise finely divided carbon particles, very finely divided catalytic particles supported on the internal and external surfaces of the carbon particles, and proton conductive material intermingled with the catalytic and carbon particles.
The catalytic particles, e.g., finely comminuted platinum, at each membrane/electrode interface induce the desired electrochemical reaction. On the anode side, the fuel (e.g., hydrogen) permeates the porous electrode material and reacts with the catalytic particles to form hydrogen cations (e.g., protons) which migrate through the ion exchange membrane to the cathode side. On the cathode side, the oxidant (e.g., oxygen-containing gas) reacts with the catalytic particles to form oxygen anions. At the cathode, the anions react with the cations to complete the electrochemical reaction and form a reaction product (e.g., liquid water).
In conventional fuel cells, the MEA is positioned between a pair of electrically conductive elements, typically plates, which serve as current collectors for the anode and cathode. The plates are often formed with channels to facilitate the distribution of the aforementioned gaseous reactants over the anode and cathode catalyst surfaces. When a plurality of fuel cells are configured as a stack to form a series electrical connection between them, the plates provide the electrical connection and are often referred to as bipolar plates. In such a configuration, each bipolar plate conducts current between the anode of one cell to the cathode of the adjacent cell in the stack.
In the PEM fuel cell environment, bipolar plates (and septums) are subject to corrosion. Therefore, in addition to having sufficient electrical conductivity to provide high performance in a PEM fuel cell, bipolar plates should also be corrosion-resistant so as to maintain adequate conductivity over extended periods of time. Graphite plates exhibit these qualities, but are generally, brittle and expensive to manufacture. Noble metals such as platinum are highly corrosion-resistant and manufacturable as lightweight thin plates, but the raw material costs for these plates would be prohibitive for many commercial applications. Lightweight metals such as aluminum and titanium and their alloys are not corrosion resistant in the PEM fuel cell environment, and contact elements made therefrom typically deteriorate rapidly, or they form highly electrically resistant oxide films on their surface that increase the internal electrical resistance of the fuel cell and reduce its performance.
Thus, a need exists for a fuel cell bipolar plate made from a non-noble, lightweight metal such as aluminum or titanium with surfaces that are protected against corrosion by an electrically conductive, oxidation-resistant barrier, coating or cladding.
In an exemplary preferred embodiment, a metal fuel cell bipolar plate is provided with a conductive multilayer coating and then with an overcoating which fills in the fine scale porosities in the underlying coating. The dimensions of the coating and the overcoating are selected so that the electrical conductivity of the bipolar plate is not compromised. The overcoating provides sealing of fine scale porosities and can be continuous if it has inherent conductivity, for example, an overcoating formed from a slurry of amorphous carbon or a suboxide of titanium. In the case of amorphous carbon, this overcoating is also hydrophobic which further prevents corrosive electrolytes from penetrating microporosities in the coating.
In another exemplary preferred embodiment, a metal fuel cell bipolar plate is provided with a conductive multilayer coating and then a chemical anodization process is employed to fill in the fine scale porosities in the underlying coating with a discontinuous overcoating which may not have high electrical conductivity, for example, an aluminum oxide, but which guides electrical charge to the coating through discontinuities in the overcoating. The filling in of the porosities prevents corrosive electrolytes from attacking the coated fuel cell bipolar plate.
In another exemplary preferred embodiment, a metal fuel cell bipolar plate is provided with a thin, graphite emulsion coating and then a layer of graphite foil is pressed over the underlying coating. The emulsion of graphite seals in microporosities present in the graphite foil. Additionally, the hydrophobic nature of the graphite emulsion coating and the graphite foil helps prevent corrosive electrolytes from attacking the coated fuel cell bipolar plate.